Axial ligand-induced high electrocatalytic hydrogen evolution activity of molecular cobaloximes in homo- and heterogeneous medium

Abstract

Three new molecular cobaloxime complexes with the general formula [ClCo(dpgH)₂L] (1–3), where L₁ = N-(4-pyridylmethyl)-1,8-naphthalimide, L₂ = 4-bromo-N-(4-pyridylmethyl)-1,8-naphthalimide, L₃ = 4-piperidin-N-(4-pyridylmethyl)-1,8-naphthalimide, have been synthesized and characterized by UV-Vis, multinuclear NMR, FT-IR and PXRD spectroscopic techniques. The crystal structures of all complexes have also been reported. The electrocatalytic activity of complexes is investigated under two catalysis conditions: (i) homogeneous conditions in acetonitrile using acetic acid (AcOH) as a proton source and (ii) heterogeneous conditions upon immobilization onto the surface of activated carbon cloth (CC). Complex 3 exhibited high electrocatalytic HER activity under both homogeneous and heterogeneous conditions. It catalyses proton reduction to molecular hydrogen in acetonitrile solution at a lower overpotential (640 mV) with a high turnover frequency (TOF) of 524.57 s⁻¹ and demonstrates good stability in acidic conditions. Furthermore, catalytic (working) electrodes are prepared by immobilizing the complexes onto the surface of activated carbon cloth (CC) for electrocatalytic HER under heterogeneous conditions. An impressive HER performance was again obtained with catalytic electrode 3@CC in 1.0 M KOH, achieving a current density of −10 mA cm⁻² at an overpotential of 262 mV. Chronoamperometric (CA) studies showed no significant decay of the initial current density for 10 h, indicating the excellent stability of 3@CC. Additionally, UV-Vis and NMR spectral studies of the recovered catalyst after electrocatalysis revealed no structural changes, demonstrating its robustness under reaction conditions.